ASP.NET Core is super awesome, especially since it plays really well with the latest deployment methods. Microsoft has even gone as far as to pre-package an optimised .NET Core Docker image with the core libraries pre-compiled for a super-fast startup. So let’s get started, but first you’ll need to install Docker and the .NET Tools on your Linux machine if you haven’t already.
Add a Dockerfile
Add a new text file called ‘Dockerfile’ (case sensitive) to the root of your project, make sure it doesn’t have any extension (such as .txt). In the Dockerfile add the following code:
COPY ./output .
ENTRYPOINT ["dotnet", "MySite.dll"]
Update the ‘MySite.dll’ reference to the name of your project with a .dll extension. Also while you’re at it, change the version number of .NET Core if you’re not using 1.1 like I am. I highly recommend 1.1 or later with Linux due to much better performance.
Build from command line
Run the following commands in the project directory:
dotnet publish -o output -c release
This will get all dependencies, compile a release build, and put the result into the ‘output’ directory. The reason we’re using a sub-directory is due to a bug in the tooling, if we use a parent relative path (../) the ‘publishOptions/include’ config setting in project.json will be ignored and you’ll be missing a chunk of your project!
Build the docker image
Now let’s get that code into a Docker image! Run the following command:
sudo docker build -t myapp .
Feel free to rename myapp to something a little more descriptive. You can also specify multiple tags, such as:
sudo docker build -t myapp -t myapp:1.0 .
It’s really up to you with regards to tagging. If you’re publishing to a repository (likely), make sure to add the applicable repository tag to the list.
Running the image
Run the following command to start up a new container:
sudo docker run -p 8000:80 myapp
Make sure to update myapp to the name you used earlier when building. Your site should now be accessible on port 8000: http://localhost:8000
You may be wondering why we forwarded to port 80, and not 5000 or whatever port you’ve specified in launchSettings.json. As part of the aspnetcore image, an environment variable is set to tell Kestrel to host on port 80. This can be overridden either in your Dockerfile, or using app.UseUrl in your Program.cs file.
Don’t connect directly to the container
Remember, the site is still running in Kestrel so it’s not magically secure by virtue of running in a container. Make sure to use a reverse-proxy such as nginx when running your site in a production environment. Microsoft are working on hardening Kestrel so that you can use it directly in the future – but we’re not there yet.
Did you mean to run dotnet SDK commands?
You might get the following error upon execution:
Did you mean to run dotnet SDK commands? Please install dotnet SDK from:
This basically means the .dll you referenced couldn’t be found. Double-check all of your paths and filenames. It’s easy to include the source instead of the output, or even save to / from the wrong directory.
Could not load <dependency>.dll
If you get dependency errors on execution, it means the output of the publish wasn’t saved to the /app folder within the container. The .dll files from the publish operation must all go directly into the root of the /app folder in the container. This means you can’t rename /app to something else.
You can get more information from:
Official MS Github: https://github.com/aspnet/aspnet-docker. Navigate to the version of .NET Core you want (1.0/jessie, 1.1/jessie, etc.). If you’re using the output of a publish, you’ll want the ‘runtime’ subfolder.
Official MS Docker Hub: https://hub.docker.com/r/microsoft/aspnetcore/. A quick overview of the base image, although it looks like MS want to spend most of their focus on the Github account.